1. Field of the Invention
This invention relates to a magnetic field generator for an MRI, a method for assembling the same, and a method for assembling a magnet unit for the same. More Specifically, this invention relates to a magnetic field generator for MRI incorporating permanent magnets, a method for assembling the same, and a method for assembling a magnet unit for the same.
2. Description of the Related Art
A magnetic field generator for MRI uses permanent magnets. The magnet used in such an apparatus have to be made up of a plurality of magnet blocks. It is very difficult to place material blocks first and then magnetize each. Thus, in actual manufacturing, the magnetized blocks must be aligned on a plate yoke so that each of the magnet blocks has a same magnetic pole facing upward.
Conventionally, when placing the magnet blocks on the plate yoke, a surface of the plate yoke is first applied with adhesive, and then magnet blocks are bonded to the surface, as disclosed in the Japanese Patent No. 2,699,250 for example.
According to such a bonding method, upper surfaces of respective magnet blocks bonded to the plate yoke surface are not flush with each other, making an uneven surface. A magnetic field generator incorporating the permanent magnets made of such magnet blocks is apt to produce ununiform magnetic field between a pair of piece poles opposed to each other. Further, pole pieces for correcting the ununiformity of the magnetic field may be tilted to produce ununiformity in the magnetic field. Generally, after a step of mounting a pair of permanent magnets to oppose each other, a step of adjustment for uniformly distributing the magnetic field is indispensable. However, if the magnet blocks are mounted according to the above method, the ununiformity of the magnetic field is so large that the adjustment becomes very time consuming with a lot of sub-steps.
Further, according to the above method of bonding the magnet blocks, the magnet blocks each having a very intense magnetism have to be placed from above, onto the upper surface of the plate yoke, making extremely difficult to fit each of the magnet blocks snugly to adjacent magnet blocks. More specifically, when mounting, each magnet block is held with a face of predetermined magnetic pole facing upward. When the magnet block is brought above the other magnet block which is already fixed onto the plate yoke, a pulling force is generated between the two. Further, when the two magnet blocks are brought in adjacency, a repelling force is generated between the two. Since the magnet block to be placed is under such intense forces, the magnet block must be firmly held for safety while being transported. For a conventional holding mechanism, it is very difficult to fit the magnet block snugly to the place of bonding efficiently against these strong forces.
A pair of magnet units thus assembled as above are then opposed to each other so the permanent magnets are faced at a predetermined distance. This process is achieved by first assembling one magnet unit, then connecting a column yoke to the magnet unit, and finally connecting the other magnet unit to the column yoke.
The column yoke is to magnetically connect the pair of magnet units, and therefore must be made of a magnetic material. Thus, when the column yoke is connected to the magnet unit, the column yoke is brought under the pulling force from the magnet unit, making difficult to connect the two at a high accuracy. Likewise, when the second magnetic unit is connected to the column yoke already connected to the first magnet unit, it is also difficult to connect the two at a high accuracy.
It is therefore a primary object of this invention to provide a magnetic field generator for MRI, a method for assembling the same, and a method for assembling the magnet unit for the same for assembling the magnetic field generator at a high efficiency through easier assembling operation of the magnet unit, easier connecting operation and other assembling operations of the magnet unit and the column yoke.
The method for assembling a magnet unit according to this invention is a method for assembling a magnet unit by bonding magnet blocks to a bonding object provided in a surface of a plate yoke, comprising: an applying step in which adhesive is applied to at least a side surface of the bonding object or a side surface of the magnet block; a transporting step in which the magnet block is transported by sliding on the plate yoke; and a bonding step in which the transported magnet block is bonded to the bonding object.
It should be noted here that the term bonding object as used here in this document refers to an object to which a new magnet block is bonded in the surface of plate yoke. For example, the bonding object may be a projection provided in the surface of plate yoke, or may be a magnet block already fixed in the surface of plate yoke.
According to the above method, the magnet block is transported by sliding to fit the bonding object, i.e. the projection or another magnet block. Then, the magnet block is held press-fitted for a predetermined amount of time to complete the bonding to a predetermined location. Since the magnet blocks are mutually connected side by side, and therefore it is not necessary to apply adhesive to the surface of plate yoke, the upper surface of the resulting permanent magnet is not likely to be uneven. Further, transportation of the magnet block to the predetermined position can be achieved simply by sliding on the plate yoke, making possible to assemble the permanent magnet stably and efficiently. A note should be made here that in order to avoid a short circuit of the magnetic flux the projection should be made of non-magnetic material such as aluminum.
Before the magnet block is transported, a first guiding member is disposed in the surface of plate yoke. Positioning of the magnet block for fixation is achieved by bringing this magnet block into contact with a side surface of the first guiding member. By this operation the magnet block is accurately placed to the predetermined position. The first guiding member may be a pair of rails laid at 90-degree angle from each other.
Preferably, a recess should be formed at a place on a side surface of the first guiding member where two blocks are bonded to each other. The first guiding member must be removed after the permanent magnet has been assembled. The recess provided in the first guiding member effectively prevents, when the magnet blocks are bonded by adhesive, the adhesive squeezed out of the mating surfaces from sticking to the first guiding member, thereby preventing the magnet blocks from being bonded to the first guiding member.
For easy positioning, a new magnet block is fitted to a corner portion formed by side surfaces of a plurality of bonding objects provided on the plate yoke, i.e. a corner portion formed by side surfaces of a pair of adjacent magnet blocks already fixed on the plate yoke, or a corner portion formed by a side surface of the projection and a side surface of the magnet block fixed directly to the projection. Especially, the placement of the magnet blocks can be achieved without misalignment if the new magnet block is transported so that a side surface of the new magnet block is held parallel to one of the side surfaces of the corner portion, i.e. the side surface of the existing magnet block or the side surface of the projection.
Likewise, when the first guiding member is used, a new magnet block is fitted to a corner portion made by the side surface of the first guiding member and the side surface of the bonding object, i.e. a corner portion formed by the side surface of the first guiding member and a side surface of the projection, or a corner portion formed by the side surface of the first guiding member and a side surface of the magnet block fitted directly thereto, for easy positioning. Especially, the placement of the magnet blocks can be achieved without misalignment if the new magnet block is transported so that a side surface of the new magnet block is held parallel to one of the side surfaces of the corner portion, i.e. the side surface of the first guiding member or the side surface of the projection.
The other method for assembling the magnet unit according to this invention is a method for assembling a magnet unit by disposing a plurality of magnet blocks with a same magnetic pole facing upward in a surface of a plate yoke, comprising steps of: disposing a magnetic member above the plate yoke at a predetermined distance from the plate yoke generally in parallel thereto; and transporting each of the plurality of magnet blocks on the plate yoke, and fixing each of the plurality of magnet blocks to adjacent magnet blocks by bonding.
When a plurality of magnet blocks are disposed with a same magnetic pole facing in a same direction as described above, the magnet blocks are apt to be demagnetized by a reverse magnetic field acting on each of the magnet blocks. To avoid this problem, when the magnet blocks are placed, a pole piece as a magnetic member is disposed above the magnet blocks. This arrangement allows the pole piece to draw the magnetic flux, reducing the reverse magnetic field, thereby preventing demagnetization of the magnet block.
Further, if the pole piece is disposed as described above, it becomes possible to accurately dispose the pole piece at a predetermined position simply by lowering the pole piece onto the permanent magnet after the magnet blocks have been fixed. The pole piece, which is a large magnetic member exerting a very strong pulling force, can be very dangerous to the safety of operation as well as very difficult to accurately position to a predetermined location if brought close to the permanent magnet after the permanent magnet has been assembled. On the other hand, if the pole piece is disposed above the plate yoke in advance, the operational safety is assured as well as accurate positioning to the predetermined place becomes possible.
The method for assembling a magnetic field generator for MRI according to this invention is a method for assembling a magnetic field generator for MRI comprising: a pair of magnet units each including a plate yoke and a permanent magnet formed in a surface of the plate yoke; and a column yoke supporting and magnetically connecting the pair of magnet units, with one of the surfaces formed with the permanent magnet opposed to the other. The method comprises steps of: disposing a second guiding member for guiding the column yoke, between the permanent magnet on the plate yoke and a location where the column yoke is to be disposed on the plate yoke; lifting the column yoke above the magnet unit; and disposing the lifted column yoke onto the magnet unit by lowering the lifted column yoke along a side surface of the second guiding member.
Preferably, after disposing the second guiding member between the permanent magnet on the plate yoke and a hole where the column yoke is to be disposed in the plate yoke for guiding the guiding rod of lifted column yoke to the hole, the positioning of the column yoke is achieved by lowering the column yoke along the side surface of the second guiding member. The step can be performed more efficiently if the side surface of the second guiding member facing the hole is formed to generally fit the side surface of the column yoke as properly positioned for example. When having lifted, the column yoke is tilted to the vertical axis due to the pulling force exerted by the permanent magnet. By bringing the column yoke into contact with the guiding surface of the second guiding member, the column yoke can be lowered easily onto the predetermined position. The second guiding member should preferably have a height not smaller than 700 mm.
Another method for assembling a magnetic field generator for MRI according to this invention is a method for assembling a magnetic field generator for MRI comprising: a pair of magnet units each including a plate yoke and a permanent magnet formed in a surface of the plate yoke; and a column yoke supporting and magnetically connecting the pair of magnet units, with one of the surfaces formed with the permanent magnet opposed to the other. The magnet unit has an upper surface formed with a hole. The method comprises steps of: lifting the column yoke above the hole; attaching a guiding rod for insertion into the hole, to a lower end face of the column yoke; and connecting the column yoke and the magnet unit through guided lowering of the column yoke by inserting the guiding rod into the hole.
According to this method, the column yoke can be disposed to a predetermined position at a high accuracy by guiding the guiding rod into the hole while the lifted column yoke is being lowered.
Still another method for assembling a magnetic field generator for MRI according to this invention is a method for assembling a magnetic field generator for MRI such as described above. One of the magnet units above the other has a hole opening at a position where the column yoke is to be connected. The method comprises steps of: disposing the column yoke on an upper surface of the lower magnet unit; attaching a guiding rod to an upper end face of the column yoke; lifting the upper magnet unit above the column yoke; and connecting the upper magnet unit and the column yoke through guided lowering of the upper magnet unit by inserting the guiding rod into the hole.
According to this method, when the upper magnet unit is mounted onto the column yoke which is already fixed to the lower magnet unit, the upper magnet unit is lifted above the column yoke, with the permanent magnet faced downward, and then the upper magnet unit is lowered and disposed on the column yoke while being guided so that the guiding rod is inserted into the hole.
During the above step, if a pair of guiding rods each having a length differing from the other are used, a preliminary positioning between the column yoke and the magnet unit can be achieved by first inserting one guiding rod into the corresponding hole, and then a more accurate positioning can be achieved by inserting the other guiding rod into the corresponding hole.
The magnetic field generator for MRI according to this invention comprises: a pair of magnet units each including a plate yoke and a permanent magnet made of a plurality of magnet blocks disposed in a surface of the plate yoke; and a column yoke supporting and magnetically connecting the pair of magnet units, with one of the surfaces formed with the permanent magnet opposed to the other. The surface of the plate yoke formed with the permanent magnet is formed with a projection, and the magnet block is bonded side by side with the adjacent magnet block or the projection. Preferably, the projection is made of nonmagnetic material.
According to this invention, the permanent magnet used for the magnetic field generator for MRI can be assembled efficiently. Further, the magnet unit and the column yoke can be assembled efficiently and accurately. As a result, the magnetic field generator for MRI can be assembled efficiently.